1. Field of the Invention
The present invention relates to inverter units, and more specifically, to inverter units which may stabilize an output from an inverter circuit, so that an output voltage does not vary even if a voltage of a power source varies, when a direct current (DC) voltage of a DC power source for use in vehicles is converted into a pulse train-like, pseudo-alternating current (AC) voltage using a plurality of switching elements.
2. Description of Related Art
For example, a DC power source, such as a battery, may be mounted on a vehicle, and a DC voltage of the DC power source is used for various purposes and controls. This DC voltage of the DC power source is converted into a pulse train-like, pseudo-AC voltage by using an inverter circuit having a plurality of switching elements. The pseudo-AC voltage is used by a motor for driving a compressor (e.g., a brushless motor of a motor-driven compressor and the like) in an air conditioning system for vehicles, or for driving other motors (e.g., a reluctance motor or an induction motor), or as an output voltage which is output to a transformer and other devices.
In a compressor, such as a refrigerant compressor, used in an air conditioning system for vehicles, although the compressor may use an engine rotational force as its drive source, such as in a motor-driven compressor or a hybrid compressor, the compressor often includes a brushless motor. This brushless motor is driven by a pulse train-like, pseudo-AC voltage converted by an inverter circuit, and the rotational speed is controlled by a method for controlling a supplied effective voltage by altering a duty cycle ratio of the pseudo-AC voltage, that is, by pulse width modulation (PWM).
Nevertheless, a DC power source mounted on a vehicle, such as a battery, may generate a voltage fluctuation in relation to an electric load. For example, in a vehicle, such as an electric vehicle or a hybrid vehicle, having a motor for running the vehicle, when accelerated; the voltage of the DC power source may decrease rapidly due to large load current flowing in the motor for running the vehicle, and when decelerated, the voltage of the DC power source may increase rapidly due to a regenerative current from the motor for running the vehicle. Further, even in a vehicle driven by an internal combustion engine, if the power of the DC power source is used in a large amount, the voltage of the DC power source may decrease rapidly.
When such a fluctuation of the voltage of the DC power source occurs, the output voltage varies proportionately thereto. For example, when the above-described, brushless motor is driven; accompanying this voltage fluctuation, the rotational speed of the motor may be accelerated or decelerated rapidly. Further, when decelerated, an excess current may flow, and in a worst case, the rotation of the motor is stopped by the operation of a protective circuit.
In response to such problems, Japanese Patent No. 3,084,941 discloses a control method, wherein a voltage of a DC power source is detected, and a duty cycle is controlled in response to the detected voltage value.
Nevertheless, when controlled by such a method, it may become difficult to obtain a desirable output stably and accurately. In particular, in the above-described, control method, a time delay due to a processing speed of the control unit, or the like, may occur between the recognition of the fluctuation of the voltage of the DC power source and the adjustment of the duty cycle, and, therefore, it may be difficult to adjust the duty cycle quickly and properly. Consequently, a fluctuation of rotational speed, excess current or lack of torque in the brushless motor, or the like, may occur.